Mass algal culture system

ABSTRACT

An apparatus and process for the culture of algae in a liquid medium is disclosed. The medium circulates through an open trough and is exposed to an atmosphere which is temperature regulated. The nutrient content of the liquid medium is regulated to control the chemical composition growth and reproduction characteristics of the cultured algae. Before it is allowed to strike the medium, sunlight is passed through a filter to remove wavelengths which are not photosynthetically active. Heat energy can be recovered from the filter.

This is a division of application Ser. No. 974,051, filed Dec. 28, 1978now U.S. Pat. No. 4,253,271.

BACKGROUND OF THE INVENTION

This invention relates to a method for growing photosyntheticmicroorganisms in liquid suspension, and specifically to the massculture of unicellular algae.

In recent years cultured algae has been recognized as a promising sourceof food and even of chemical feedstocks. As a result, a variety ofapparatuses and methods have been devised to facilitate the growth ofalgae. High yields have been obtained in some tightly controlledlaboratory experiments, but heretofore, efforts at mass algal culturehave been disappointing in that they were inefficient and uneconomical.

Prior mass algal culture systems have yet to prove economical because(1) they require relatively deep containment (20-100 cm) in order toprovide for temperature control; (2) they produce comparatively dilutecultures; (3) they make inefficient use of carbon dioxide and little useof infra-red radiations from sunlight; (4) they require substantialenergy inputs to provide mixing to avoid thermal stratification; (5)they must process larger volumes of water to obtain the same harvestyields of algal matter that might be collectible from shallower systems;and (6) they permit little or no control and/or regulation of thoseenvironmental elements which control and regulate the performancecharacteristics of the cultured cells.

Further, no prior mass algal culture system has been equipped to induceand regulate the flashing-light effect efficiently. Prior apparatuseshave suffered from rapid contamination by unwanted organisms and haverequired extensive sterilization treatments for both equipment andmedia. Nutrient and temperature management has not been conducted withprecision in such mass culture systems.

Harvesting is usually the single largest deterrent to realizingpractical and economical unicellular algal production. The usual methodsemployed include settling, perhaps enhanced by floculation,centrifugation, and bed evaporation. All such processes require too muchtime, space, and/or energy to permit reasonable commercial utility.

Even the best of existing apparatus have been operated at less than peakefficiency because currently known methods of operation are notregulated to maximize the production of cell matter.

SUMMARY OF THE INVENTION

It has now been discovered that the deficiencies of existing systems canbe overcome by the use of novel apparatus and processes which permit asubstantial gain in the net energy (outputs vs. inputs) obtained fromthe system, without being substantially more complex to operate thansystems heretofore used. The mass culture apparatus disclosed canprecisely regulate many variables so that the cells harvested can becontrolled to be of chosen chemical compositions and produced at ratesrepresenting high and nearly constant conversion efficiencies ofsunlight into stored chemical-free energy. In this way, the algalproduct can be chosen to meet a variety of needs.

As in some prior systems, algae is cultured in a liquid medium whichflows through a shallow trough. In the present system, the trough ispositioned beneath a filtering means which absorbs the infrared andultraviolet wavelengths of sunlight passing therethrough. Algal cells inthe liquid medium thus receive light of the photosynthetically activewavelengths which stimulate growth and reproduction, but are not exposedto substantial amounts of light of wavelengths which retard or are notused in those life functions. The captured wavelengths heat thefiltering means which therefore functions as a solar collector. The heatenergy developed can be used to control the temperature of the algalculture medium or can be converted into electrical energy for drivingpump motors and other essential system components.

The algae-containing liquid medium moves through a channel or trough bygravity flow. A pump removes medium from a discharge end of the troughand redeposits it in an inlet end of the trough. A gas lift pump isuniquely advantageous for this purpose because such a pump not onlycirculates the liquid medium but also can be used to separate organicwastes from the medium. A stream of minute gas bubbles can be injectedinto the liquid medium 12 as it passes through the gas lift pump. Thebubbles possess a static charge so that organic wastes in the liquidmedium become attached to oppositely charged bubbles. The bubbles riseto the surface of the liquid medium carrying the electrostaticallyabsorbed organic substances with them. When at the surface, the bubblesform a froth. This froth and the undesirable organic substances itcontains, may then be easily separated from the liquid medium.

Channels of the present invention can be arranged in a serpentinepattern. Liquid medium containing growing algae is circulated throughthe serpentine channels at a rate sufficient to cause mixing orturbulence therein, thereby to achieve a desired periodicity of thefluid element such that its components are alternately given access to asurface layer of the fluid and deeper layers therein. Individual algalcells, following the flow pattern of the liquid medium, are transportedcontinuously between surface locations and regions deeper within thechannels. Because the overlying algal cells are continuously andcyclically exchanged with those deeper within the medium, and becausethey extinguish light by absorption and scattering in direct proportionto their concentration in the medium, they progressively shade the cellsto the point of virtual darkness in the deeper zones within thechannels. Thus, individual cells growing within the shallow (2-5 cmdeep) fluid element are exposed to alternating periods of light anddarkness and exhibit the desirable growth characteristics associatedwith the phenomenon commonly known as the "flashing light effect"throughout most of the channel system.

Near its discharge end, the channel deepens and widens so that thecross-sectional flow area is increased. As a result, laminar flow isestablished within the fluid element as it approaches the discharge end.

Flow is regulated so that near stagnant conditions are produced in thesurface waters at the discharge end whereby algal cells within the fluidelement tend to rise to the surface, forming a thick surface film. Thisalgal film is readily harvested either by regulating its flow over awier or by other skim harvesting techniques. The efficiency of thesetechniques is improved by processes which increase the flotation of thealgal cells. Such processes include (1) the electrostatic attachment ofalgal cells to gas bubbles within the fluid element due to theproduction of minute bubbles in the gas-lift pump and/or the naturalformation of oxygen bubbles by the cells during photosynthesis, (2) themodification of the chemical composition of the algal cells by judiciousselection and regulation of those environmental factors which direct thebiosynthesis of chemical compounds less dense than the growth medium,and (3) the addition of surfactants which are less dense than water andwhich adsorb to and increase the flotation of the algal cells.

Prior to innoculation with an algal culture, the apparatus is sterilizedby preparing a liquid medium precursor containing all the acidiccomponents of the desired liquid medium. The trough is filled with thisprecursor to kill any potentially contaminating organism. An alkalinesubstance is then added to the precursor to complete the liquid mediumand raise its pH to within a range suitable for the growth of algae. Themedium is innoculated with the desired algae and maintained in anenvironment conducive to growth and reproduction.

During operation, contamination is reduced because the channels arecontained within an enclosure, in which a somewhat elevated pressure ismaintained. The system is provided with means to carefully regulatenutrient and carbon dioxide content of the liquid medium and to maintainthe medium within preferred temperature and pH ranges. In this way thesystem affords an output of algal products having more uniform andcontrollable chemistry, adjustable over a greater range of desirablecompositions, than have heretofore been obtained in mass culturesystems.

By appropriate selection of nutrient schedules it is possible tomaximize cell reproduction, cell enlargement and/or the concentration ofcertain chemical compounds, such as lipid constituents, in the algalproduct. Continuous operation can be achieved by continuously orperiodically adding nutrients to the medium to make up for thoseconsumed by the growing algae and by recycling liquid medium until suchtime as the liquid becomes contaminated with undesirable organisms orwith a toxic level of some algal secretion which can not be removedsatisfactorily.

It is therefore an object of this invention to provide an algal culturesystem wherein algal cells are exposed to light which is chieflycomprised of photosynthetically active wavelengths and wherein algalcells are grown in shallow media (2-5 cm) and sheltered from light ofwavelengths which inhibit growth and reproduction.

A further object is to provide such an algal culture system whereinfiltering means absorb essentially all radiation below 350 nanometersand above 700 nanometers from incident sunlight.

It is a further object to provide a system wherein the temperature ofthe algal culture is regulated to provide an ideal growth environment.

An additional object is to use the nonphotosynthetic wavelengths oflight to provide energy for nonphotosynthetic operations of the system.

Another object is to provide means of bringing potentially toxicdissolved organic materials into contact with gas bubbles so that theywill float to the surface of a liquid culture medium where they can beconveniently removed.

An additional object is to provide a gas lift pump mechanism which bothmoves liquid culture medium through a trough and brings algal cells intocontact with gas bubbles.

Another object is to provide a gas lift pump as aforesaid to regulateand control the build up of potentially toxic dissolved organicmaterials.

An object is also to provide an algal culture system wherein algal cellscan be continuously grown and harvested.

Yet another object is to provide an algal culture apparatus whereinalgae growing in a liquid medium are alternately exposed to periods oflight and darkness to obtain the favorable growth characteristicsinduced by the "flashing light effect."

In addition, it is an object to provide an algal culture system asaforesaid which is of simple construction and which stores sunlight aschemical energy in excess of the amounts required for system operation.

Another object is to provide a mass culture system wherein an algalnutrient medium can be regulated to control uniformly the chemicalmakeup of the harvested algal product and to achieve routinely a greaterproductivity than has heretofore been practical.

An object is to provide a mass algal culture process capable ofregulating the physiological characteristics of algae grown andreproduced therein.

It is also an object to provide a method of liquid culture mediumformulation wherein a liquid precursor of the culture medium acts as asterilizing agent for the culture growth environment.

A further object is to provide an efficient outdoor system for storingsolar energy in chemical form represented by the mass culture of marinealgae in a seawater based medium.

These and other objects and features of the present invention will beapparent from the drawings and description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective view of an algal culture system in accord withthe present invention with its walls partially broken away;

FIG. 2 is a plan view of the algal culture trough and part of theassociated apparatus of FIG. 1 with the walls of the enclosure andcertain other parts shown in horizontal section;

FIG. 3 is an enlarged partial sectional view taken along line 3--3 ofFIG. 1;

FIG. 4 is a sectional view taken along line 4--4 of FIG. 3;

FIG. 5 is an enlarged partial sectional view taken along line 5--5 ofFIG. 2;

FIG. 6 is a partial sectional view taken along line 6--6 of FIG. 2showing desired flow vectors of liquid medium approaching the outlet endof the trough.

FIG. 7 is a sectional view taken along line 7--7 of FIG. 1;

FIG. 8 is a schematic diagram of the algal culture system's solar heatrecovery system.

FIG. 9 is a schematic view showing the preferred sinusoidal path of atypical algae cell as it moves through an algal culture trough of thetype shown in FIG. 1.

FIG. 10 is a partial plan view of an algal culture trough containingmixing riffles;

FIG. 11 is a partial sectional view taken along line 11--11 of FIG. 10;and

FIG. 12 is a schematic top view of a branched "one-pass" algal culturetrough according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred apparatus for the production of algae according to thepresent invention is shown in FIGS. 1, 2 and 7. A trough 13 is providedto contain a liquid algae culture medium 14 in a shallow layer mostpreferably two to five centimeters deep. The trough has an inlet end 15where the liquid medium is introduced and a discharge end 16 whereliquid medium is withdrawn. The trough may be level or slightly inclineddownwardly, e.g. at about one percent, toward the discharge end 16 tofacilitate the flow of liquid medium therethrough by gravity.

Legs 18 support chamber walls 20, a floor 24 and a substantiallyoptically transparent ceiling member 26 which together comprise anenclosure 28 for the trough 13. The enclosure defines an interior gasspace 29 wherein an appropriate gaseous atmosphere may be maintained ata slightly elevated pressure so that airborne contaminants can not enterthe enclosure. The walls 20 and floor 24 are preferably opaque so thatincident light can enter the gas space 29 only through the ceilingmember 26. With this arrangement, light and heat are the only externalelements which effect the environment inside the growth apparatus.

Although nonrecirculating trough systems may be used as described below,the embodiment shown in FIGS. 1 to 7 is a recirculation system whereinthe liquid medium 14 makes multiple trips through the trough 13. Whenthe liquid medium 14 reaches the discharge end 16 of the trough, it isrecirculated to the inlet end 15 by means of a suitable pump. Preferredis a gas lift pump mechanism 30 shown in FIG. 3. This pump includes asubstantially vertical stand pipe 32 having a medium inlet 34 and amedium outlet 35 located above the inlet.

A stream of gas is injected into the pipe 32 through an inlet opening 36shown in FIG. 3 as being located below the medium outlet 35. As the gasbubbles rise in the stand pipe 32, they carry liquid medium in the pipeupwardly and into the trough inlet end 15. In the illustratedembodiment, gas from a source such as an air compressor 37 or acompressed air cylinder (not shown), delivered to the inlet 36 via a gasdelivery tube 38.

The inlet of the gas lift pump is connected by a recirculation pipe 39to the discharge end 16 of the trough; and the outlet 35 of the gas liftpump is positioned to deposit liquid medium from the pump into the inletend 14 of the trough. With this arrangement the pump 30 can be used tocontinuously recirculate algal culture medium through the trough 13, bywithdrawing liquid from the trough's discharge end 16 and replacing itin the inlet end 15. Recirculation through line 39 is accomplished at asteady rate controlled by the rate of air addition to the pipe 32 fromair compressor 37. A check valve 40 prevents the circulating culturemedium from backing into the air delivery tube 38 during operation andpump downtimes.

The gas lift pump mechanism shown in FIG. 3 includes several additionalfeatures. The pump mechanism includes means for regulating liquid mediumtemperature throughout a wide range of temperatures between freezing andboiling. This feature is included because each species of algae developsmost efficiently in a narrow range of temperatures. To maintain optimumgrowth temperatures the gas lift pump is equipped with jacket 41 whichsurrounds a portion of the piping through which the medium flows. FIG. 3shows such a jacket surrounding the pipe 32. When the liquid culturemedium 14 drops in temperature to below a desired range, a hot liquidmay be circulated through the jacket 41 to elevate the temperature ofthe medium. Likewise, an overheated medium will give up heat to arelatively cold liquid circulating through the jacket 41. Thermostaticcontrols (not shown) may be provided to automatically operate this heatexchange mechanism for maximum efficiency. Heat exchange means forregulating medium temperature could, of course, be provided at otherlocations along the liquid medium's path as will more fully be describedherebelow. Resistance heaters could also be located along the trough 13to heat the medium.

The illustrated gas lift pump is further preferred because it includesintegral means for injecting special purpose gases into the liquidmedium. Although other means could be provided for this purpose, thestructure of the illustrated pump mechanism 30 is unique and especiallyadvantageous.

Gas cylinders 43 or other gas sources are each connected by a valvedfeeder line 44 to a special gas supply line 45. The line 45 connects tothe pipe 32 at a location beneath the medium inlet 34. A porous element37 shown in FIG. 3 is located inside the pipe 32 between the mediuminlet 34 and the gas supply line 45 to disperse gas admitted through thesupply line into bubbles. The illustrated element comprises upper andlower perforated plates 48, 49, each having uniform hole patternsidentical to one another. Bubble size is selected by regulating thedegree of pore overlap between the two adjacent plates. Pore overlap isvaried by twisting a ring structure 50 to which the lower plate 49 isattached with respect to the upper plate 48 which is fixedly mounted inthe pipe 32. Other constructions such as a block of microporousmaterial, could also be used for the element 47.

Because the porous element 47 disperses gas into a stream of bubbles inthe liquid medium, carbon dioxide advantageously can be added to themedium through the element. Carbon dioxide is an essential compoundconsumed by algae during photosynthesis; and in a mass algal culturesystem, it is rapidly consumed from liquid culture medium and must bereplaced. Preferably, at least 235 grams of carbon dioxide should bereplaced per each hundred liters of medium per day. Carbon dioxide fromstack gasses, process gas or a gas production plant may conveniently beused. When the carbon dioxide is bubbled into liquid medium 14circulating through the pipe 32, the high surface area of the bubblesand the long period of liquid contact facilitates efficient dissolutionof the carbon dioxide. Carbon dioxide can thus be added to the medium atrates required to maintain a desired medium alkalinity and nutrientcarbon content. Because carbon dioxide is admitted to the gas lift pumpthrough an inlet separate from the circulating gas inlet 36, the rate ofcarbon dioxide addition is easily controlled independently of the liquidmedium circulation rate and can be established to minimize the escape ofheat-retaining carbon dioxide gas into the gas space 29.

To retain sufficient amounts of carbon dioxide in a liquid culturemedium, the pH of the liquid should be maintained within an appropriaterange. For fresh water media wherein carbon dioxide is retained as adissolved gas, a pH of between 6.0 and 7.5 is preferred. A range of 7.5to 9.5 is best for salt water media wherein carbon dioxide is present asdissolved bicarbonate ions.

Substantially water insoluble gasses such as oxygen, hydrogen and ozonemay also be delivered from one of the tanks 43 into the pipe 32. Bubblesof such gasses, produced by the porous element 47, carry a static chargewhich is determined by the chemical natures of the gas added through theline 45 and of the liquid medium. The static charge makes the gasbubbles attractive to oppositely charged substances in the liquid mediumand the two tend to adhere to one another due to electrostaticadsorption. After the liquid medium 14 is carried from the pipe 32 intothe inlet end 15 of the trough, the entrained gas bubbles tend to riseto the surface carrying with them the adhering substances.

Small, lightweight particles, such as organic waste substances tend toadhere to oppositely charged gas bubbles and to be carried to the mediumsurface in this manner. When bubbles carrying the organisms reach thesurface, a froth tends to form on the surface of the medium 14 near theinlet end 15. By providing an appropriate foam separation device nearthe inlet end, the froth containing undesirable organic substances canbe separated from the liquid medium.

One suitable foam separation device 52 is shown in FIGS. 1 and 5. Inthis device the stand pipe 32 of the gas lift pump extends upwardlythrough the floor of trough 13. Inside the trough, a drum 53 surroundsthe pipe 32 and is provided with a liquid discharge slit orifice 54located below the uppermost end of the pipe 32. A gate 55 is provided toregulate the fluid head inside the drum 53, by adjusting the size of theorifice 54. A collector tray 56 surrounds the top of the drum 53; and achannel flume 57 extends downwardly from the tray.

The previously described froth, which contains organic materials,collects on the surface of liquid medium inside the drum 53 as theliquid medium is continuously discharged through the orifice 54 into thetrough 13. As the froth collects, it rises and eventually flows over thetop of the drum into the tray 56 from which it is discharged through theflume 57.

Other devices, such as the skimming device hereinafter described, couldbe used for removing froth from the surface of liquid medium 14 in thetrough 13.

Certain water insoluble gasses injected through the porous element 47may also have an affinity for algal cells. If the rate of charged gasinjection is sufficiently fast, bubbles of gas may adhere to cellspassing through the pipe 32. Once carried into the drum 53, the algalcells tend to rise to the surface. Due to their relatively large size,the cells do not rise as rapidly as the bubbles bearing organicsubstances, but instead are caught up in the subsurface flow of liquidmedium and carried through the orifice 54 into the trough 13.

The relative attraction of algal cells and organic substances to the gasbubbles can be regulated by selection of the types and amounts of gasinjected as minute bubbles, by regulating the size of the bubbles and byadjusting the flow rate of liquid medium and the injection rate ofgasses. To prevent algal cells from being removed in the foam separationdevice 52, the above factors may be selected to minimize algal cellflotation and maximize organic material flotation inside the drum 53.

A skimming device 59 is located in the trough 13 for continuouslyharvesting algal cells. One skimmer means suitable for this purpose isshown in FIGS. 1 and 6. This device comprises a porcelain drum 60 whichmay be lowered so that the lowermost portion of the drum extends justbeneath the surface of the liquid medium 14. The drum thus provides asurface barrier without substantially impeding subsurface flow of theliquid medium. As liquid medium moves downstream in the direction ofarrows 61, it passes beneath the drum 60. The drum 60 is rotated(counterclockwise in FIG. 6) so that the rising face of the drum facesupstream. As the drum rotates, floating algal cells adhere to the risingface of the drum 60 and are carried upwardly over the drum. A doctorblade or squeegee 62 is positioned against the drum 60 to scrape thealgal cells from the drum surface and into a discharge chute 63. Theskimming device 60 may conveniently be located in the trough 13 near thedischarge end 16. It could, however, be located at any position alongthe trough 13.

It is advantageous to bring as many algal cells as possible to thesurface of the liquid medium so that they are accessible for skimharvesting. Flotation of algal cells can be enhanced by any meanscapable of slowing the flow rate and reducing the mixing of liquidmedium ahead of the harvester so that the pull of the liquid stream isovercome and the naturally buoyant algal cells rise to the surface. Inthe illustrated embodiment, flow is reduced at the discharge end 16 ofthe trough by regulating the rate at which liquid is removed through thepipe 39. The gas lift pump 30 is operated at a constant rate such thatthere is always a semistagnant buildup of medium at the discharge end16.

The flotation of algae is further enhanced because the channel bothwidens and deepens as it approaches the trough's discharge end 16. Thiscauses a laminar flow pattern to be established in the liquid mediumcommencing somewhat upstream of the skimmer 60. FIG. 4 shows flowvectors (represented by arrows) observed at the discharge end of thepreferred channel. It can be seen that increased cross-sectional flowarea causes transition from transitional and/or turbulent flow tolaminar flow.

The drum 60 of the skimmer 59, acts as a gate which reduces the flow ofsurface liquid further so that a near-stagnant surface layer of mediumis maintained upstream of the skimmer 59 as well as between the skimmerand the discharge end 16. Adjustment of the drum's skimming actionregulates the discharge of surface water in the region ofnear-stagnation.

Upstream of the discharge end 16, the liquid medium is subjected tocontinuous mixing as will be described hereinafter. This mixing tends tomaintain algal cells in suspension. When the algal cells reach theregion of laminar flow, cells are freed from the mixing action and thosehaving densities lower than the medium tend to rise to the surface wherethey form a thick film in the near-stagnant surface layer. Flotation ofalgal cells can further be enhanced by regulating the growth environmentin the trough 13 to maximize the cells' biosynthesis of chemicalcompounds less dense than the liquid medium 14. Surfactants less densethan the medium 14 can be added to the medium upstream of the dischargeend 16. The surfactants adsorb to and increase the flotation of thealgal cells.

Once on the surface, algae may be skimmed off readily by the porcelaindrum 60 and constitute the end product of the system.

As an alternative to skim harvesting, surface algal cells at thedischarge end 16 can be harvested from the liquid medium by a conductingregulated flow of liquid medium over an end channel weir (not shown).Control of flow over such a weir regulates the discharge of surfacewaters within the region of near-stagnation.

Harvesting may be accomplished most conveniently during daylight hoursbecause algal cells emit oxygen during photosynthesis. The oxygencollects in small bubbles which cling to the algal cell wall and thusincrease the flotation of the cells. Flotation is also enhanced by theminute bubbles which are formed in the airlift pump mechanism 30 andwhich may remain attached to the cells up to the time of harvesting.

Other devices for continuously harvesting algal cells, such as rotaryscreens or hydroclones, may be used. A skimmer is preferred, however,because such a device is highly efficient when used as described above.

To avoid contamination of the liquid culture medium 14 by undesirableairborne species, it is advisable that the gas space 29 be sealed fromthe surrounding atmosphere or be maintained at a slight positivepressure so that gas will tend to flow out of the space 29 to thesurrounding atmosphere. One suitable way to avoid contamination is bycontinuously pumping a stream of filtered air into the space 29 andallowing any excess gas to flow out of the chamber. Some gas is, ofcourse, added to the space from the supply lines 38, 45 via thestandpipe 32. In addition, it may be helpful to inject cool, filteredair taken from the surrounding air to reduce the temperature of gascontained in the space 29. This prevents gas in the space 29 frombecoming overheated which could adversely affect algal production.

To further enhance the growth of algae, this apparatus is provided withmeans for filtering radiation of undesired wavelengths from incidentsunlight. One suitable filtering means is a shallow container of lightfiltering liquid 64. Such a container, as shown in FIGS. 1 and 6, may beof "sandwich" construction, including the ceiling member 26 which servesas a lower panel of the container, a spaced upper transparent panel 65and walls 66 joining the perimeters of the two panels to define awatertight compartment. The compartment contains a layer of liquid 64which is selected for its ability to filter undesired wavelengths fromthe sunlight before it enters the gas space 29. If algae is to growproperly in the system, it is necessary that the panels 26, 65 be madeof a material which is substantially transparent to those wavelengths ofsunlight which are photosynthetically active. Preferably the panel 65will have a flat face inclined toward the sun at an angle of up to about60° from horizontal to avoid reflection loss of photosyntheticallyactive wavelengths.

A variety of different liquids might be suitable for use in thecontainer depending upon their light absorption spectra. One especiallysuitable liquid is an aqueous solution of CuSO₄. An effective solutionwill contain about five to ten weight percent CuSO₄.5H₂ O divided by thelength, in centimeters of the light's path inside the solution. Such asolution layer will trap the ultraviolet and infrared wavelengths whichinhibit algal growth and/or normally would be unutilized.

A radiation absorbing gas or gel could be used in the container 62 inplace of the liquid solution. A gas suitable for this purpose is amixture of ammonia and sodium thiocyanate. Alternately, filtering can beachieved using a solid filter plate. If, for example, one of thecontainer panels 26, 65 is impregnated with copper sulfate salt, atransparent liquid or gas could be circulated between the panels toreceive heat energy absorbed from solar radiation by the impregnatedpanel. As a filter, the impregnated panel will suffice alone if recoveryof heat energy is not desired.

Regardless of filter type, the filter should be capable of transmittingphotosynthetically active wavelengths and at the same time absorbing orreflecting at least ninety-nine percent of all incident radiation below350 nanometers and above 700 nanometers.

Because the air in the gas space 29 will begin to receive excessive heatduring daylight hours from the filter if the filter's temperatureincreases to an undesirably high level, a pane of transparent material(not shown) can be placed a small distance beneath the ceiling member 26to form a compartment. A layer of air trapped in such a compartmentwould thermally insulate the air space 29 from the filter. Cool aircould be circulated through such a compartment to further prevent thetransfer of heat from the filter into the air space 29.

When using a liquid solution as a filtering medium, heat can be removeddirectly from the liquid 64 to prevent overheating inside the air space29. FIG. 8 shows schematically a heat exchange system both forcontrolling the temperature of liquid 64 and for utilizing heat energycollected in the liquid. A supply of the liquid 64 is maintained in areservoir 72. A stream of the relatively cool liquid in the reservoir 72is pumped by pump 74 through a distribution tube 76 and into thecompartment between the panels 26, 65 of a "sandwich-type" solarcollector 77 as described above. Preferably, the tube 76 connects at thelowermost part of the compartment so that liquid 64 is injected alongthe lowest sidewall of the container, but other flow patterns could beused.

As the pump 74 continues to operate, the injected liquid flows upwardlybetween the panels 26, 65 collecting heat. The heated liquid flows outof the upper end of the compartment through a tube 78 which is connectedto a heat exchange unit 82. Inside the unit 82, excess heat is removedfrom the stream of filtering liquid by indirect heat exchange with acooling fluid, preferably water, or some other non-toxic fluid having ahigh heat capacity. The fluid is circulated through a heat exchange coil84 whereby it receives heat energy from the liquid 64. The temperatureof the cooling liquid is preferably thermostatically controlled tomaintain the filtering liquid at a decreased temperature. After it iscooled in the heat exchange unit 82, the filtering liquid 64 is returnedto the reservoir 72 through a return tube 85. If heating of the liquid64 is required at any time, a heated fluid can be circulated through thecoil 84.

Heat energy recovered in the exchange unit 82 can be used in otherprocess applications, stored for heating use during cold periods or, ifof sufficient quantity and quality, converted into electrical energy.The heated cooling fluid can be pumped from the heat exchange unit 82into a distribution line 86 by a pump 87. The distribution line connectsto an inlet pipe 88 of the previously described jacket 41. An outletpipe 89 returns cooled fluid to the heat exchange unit via a collectionline 90.

Another device which can utilize the heat in the cooling fluid is atrough heating system 92. The system includes temperature control lines94 which are connected between an input manifold 96 and an outputmanifold 97. As best shown in FIGS. 1 and 7, the lines 94 are embeddedin a layer of sand or similar material 98 in the bottom of the trough13. A layer of flexible sheeting 100, e.g. of polyvinyl chloride orbutyl rubber, covers the sand 98 and comprises the floor of the channeldefined by the trough 13. When the temperature of liquid medium 14 inthe trough 13 descends below a desired level, fluid heated in the heatexchange unit 82 can be diverted through the input manifold 96 into thetemperature control lines 94. Heat from the fluid thus defuses into thesand 98 to warm liquid medium 14 in the trough 13. The fluid is returnedto the collection line 90 via the output manifold 97.

Heated fluid can be diverted to other process apparatus, representedschematically in FIG. 8 as a box 102. Such apparatus might include meansfor converting heat energy into electricity for powering pumps and otherequipment.

The apparatus of the present invention further includes a device toinduce mixing of the liquid medium 14 as it passes through the trough13. The illustrated embodiment includes a serpentine trough having aseries of curves positioned at intervals. Liquid medium is circulatedthrough the trough at a rate sufficient to induce controlled mixingtherein. As a result vertical eddies are induced and the liquid mediumfollows a vortical path. In this mixing pattern algal cells periodicallymove between surface layers of the medium and layers near the bottomthereof. Periodic mixing in turn reduces settling and early flotation ofalgal cells.

Periodic or continuous medium mixing is further useful because plantsgrow most efficiently when subject to alternating periods of light andrelative darkness. Because algal cells extinguish light by absorptionand scattering in direct proportion to their concentration in themedium, relatively little sunlight penetrates to the deepest portion ofthe liquid medium 14 as compared to the amount of light available justbelow the surface. The above described periodic mixing thus causes algalcells to move alternately between well lighted positions (adjacent tothe surface) and shaded areas (distant from the surface). This in turncauses the cells to exhibit the desirable growth characteristicsassociated with the phenomenon commonly known as the "flashing lighteffect." Preferably, the trough 13 is designed to produce periodicculture mixing such that individual algal cells follow a substantiallysinusoidal path, as shown by the arrows in FIG. 9, during their travelfrom the inlet end 15 to the discharge end 16.

To obtain the most efficient use of available sunlight, the troughsystem should contain the liquid medium in a relatively thin layer,between about 0.5 and 5.0 centimeters in depth and more preferablybetween 2.0 and 5.0 centimeters. Maintaining such a shallow layerfacilitates culture mixing for the purpose of inducing the "flashinglight effect" phenomenon.

When operating a recirculating system with liquid medium in a 0.5 to 5.0centimeter depth range, the "flashing light effect" is maximized if thealgal culture density is maintained so that at all depths, lightintensity is extinguished between one hundred and one thousand foldmultiplied by the liquid medium depth in centimeters. In other words,the culture density will be in the most proficient range if lightintensity at a given depth multipled by the depth in centimeters gives afigure between about 0.1 and 1.0 percent of the light intensity at thesurface of the liquid medium.

If a portion of the algal culture is not recirculated with the liquidmedium, culture density at the inlet end of the trough may be at adensity below the above specified amount. The proscribed culture densityshould, however, be reached at some point along the trough so that algalgrowth thereafter will be progressively enhanced by optimum "flashinglight" conditions.

While the serpentine trough of the illustrated device is quite suitablefor moving cells in a vortical path, other options are available. Itmight, for example, prove desirable to culture algae in a long straighttrough. Appropriate mixing will automatically result if a suitableliquid flow rate is maintained. Mixing can further be created bypositioning baffles or other mixing devices at spaced intervals in atrough. Such devices can provide the turbulence necessary to obtain amixing pattern of the type illustrated by arrows in FIG. 9. Othertechniques well known in the art can be applied to generate the desiredturbulences.

One especially suitable mixing device is a cylindrical riffle 104 asillustrated in FIGS. 10 and 11. Such riffles may be located at intervalsalong the trough's channel bottom to create turbulence and thus culturemixing in the flowing liquid culture medium 14. The greatest amount ofturbulence occurs just downstream of a riffle and recedes thereafter.Riffle spacing should thus be set so that a riffle is located at eachpoint where turbulence induced by the immediately proceeding riffle hasdied down.

To make up for nutrients which are consumed by the algae during theirgrowth and to regulate the pH of the nutrient medium, the apparatus ofthe present invention includes lines 106 shown in FIG. 1 as extendingfrom a plurality of tanks 108 containing solutions of make up nutrientsand/or liquids for adjusting the pH of the liquid medium 14. Make upnutrients from the tanks are pumped into the trough 13 via the lines 106to replenish the liquid medium 14. A plurality of such nutrient make upand/or pH adjustment lines may be positioned at intervals along thetrough to add nutrients or liquids to adjust pH wherever needed. Thelines can also be used to add water to the trough 13 to make up forlosses due to evaporation and to add carbon dioxide to the liquidmedium. Periodic sampling and testing of the liquid medium can be usedto determine the amount of nutrients, pH and salinity adjustingmaterials, carbon dioxide or water to be added through each nutrientmake up line 106. A variety of alternative means for monitoring andreplenishing the nutrient medium 14 will be apparent to one skilled inthe art.

By careful regulation of the nutrient medium, it is possible touniformly control the quality and chemical composition of the ultimatealgal product. This is because algal cells of a single species are foundto have physiological characteristics which differ depending upon theenvironment in which they are cultured. Factors affecting the algaeinclude light wavelength and exposure timing, chemical composition ofthe nutrient, concentration of cells in the culture, and temperature.Each such factor is carefully controlled by the above described system.

It has been found, for example, that maximum lipid production can beachieved if the nitrogen concentration of a culture medium is reducedwhen the culture approaches its maximum cell population. Thus, byregulating the nutrient medium it is possible to control the lipidcontent of the product algae.

OPERATION

The basic operation of the present invention will be apparent from theforegoing description of the apparatus. The trough 13 is filled withliquid culture medium and innoculated with algae. The airlift pump 30 isactivated by introducing gas through the line 40. This causes the liquidmedium, containing rapidly growing algal cells to circulate through thechannels as a shallow layer overlying the flexible sheeting 100. Theflow rate is selected to induce the desired mixing of liquid as it flowsthrough the channels.

While the system operates, the thermostatically controlled heatexchanger 82 maintains the temperature of the filtering liquid 64 at adesired level. Also, a fluid at an appropriate temperature is circulatedthrough the jacket 41 and/or temperature control lines 94 to maintainthe liquid medium within an efficient operating temperature range.Carbon dioxide is continuously fed to the liquid medium 14 through theline 45 of the gas lift pump mechanism and/or the lines 106. After aninitial operating period, samples of the nutrient material are takenand, if the samples indicate that the nutrient level has dropped below adesired minimum, makeup nutrients are added through line 82 to promotefurther algal reproduction and/or cell growth. Once the algae hasreproduced to a desirable concentration and cell size, the harvestingmechanism 59 is lowered into the liquid 14 and the drum 60 activated tocommence harvesting.

The continuous growth of high-lipid algal cells in recirculating liquidmedium can be accomplished by establishing coordinated harvesting andnutrient addition schedules as follows. The culture is established in afull strength liquid culture medium. As the culture grows, nutrients areadded periodically to maintain their concentrations within theirdesirable range. When the rate of cell reproduction reduces as the cellconcentration approaches its limit, as determined by an electronicparticulate cell counter or similar device, nitrogen compounds areeliminated from the makeup nutrients until the nitrogen content of theliquid medium drops to about fifty percent of the amount initiallypresent in the complete medium. Subsequent nutrient additions areadjusted to maintain nitrogen at the fifty percent level while othernutrients are at full strength.

After a period at these conditions, total cell mass of the culture willhave increased, especially the lipid content thereof. Harvesting is thencommenced until about fifty percent of the cells are separated from theliquid medium. Once this has been done harvesting is stopped, thenitrogen content returned to full strength and the entire processrepeated.

It is desirable that both the trough 13 and the liquid medium 14 besterilized before use. This is especially true where natural seawater isa component of the liquid medium. If such a preliminary step is nottaken, the liquid medium may be contaminated with undesirable organisms.The present system provides a unique method of self-sterilization aswill be apparent from the following example.

EXAMPLE

A culture of Phaeodactylum tricornutum was grown in an apparatus of thetype previously described. This particular marine algae was selectedbecause of its unique growth characteristics and usefulness as an endproduct. It is further advantageous for cultivation because it has beenobserved to secrete substances having antibiotic activity. Suchsecretions may inhibit the growth of bacteria in the liquid nutrientmedium. Because Phaeodactylum tricornutum does not require externalsupplies of silica or vitamins, these nutrients can be excluded from theliquid medium to inhibit the growth of contaminating microorganismsrequiring these nutrients.

A variety of liquid mediums might be used successfully, for the cultureof Phaeodactylum tricornutum, but exceptional results are achieved usinga medium which includes sea water in which is dissolved a mixture ofnutrient salts which, in grams per 100 liters of sea water, comprises38.0-63.4 g. HNO₃, 5.9-9.8 g. H₃ PO₄, 3.2-5.4 g. KCl, 1.40-2.33 g. Na₂EDTA (ethylene diaminetetraacetic acid disodium salt), 0.015-0.025 g.FeCl₃ ·6H₂ O, 0.00275-0.00460 g. (NH₄)₆ Mo₇ O₂₄ ·4H₂ O, 0.0008-0.0013 g.H₃ BO₃, 0.00030-0.00050 g. ZuCl₂, and lesser amounts of both CuCl₂ ·2H₂O and MnCl₂ ·4H₂ O, and an amount of KOH sufficient to raise the pH ofsaid solution to within the range of 7.6-7.8.

An experimental liquid nutrient medium was accordingly prepared from 100liters of sea water and 50.7 g. HNO₃, 7.8 g. H₃ PO₄, 4.3 g. KCl, 1.86 g.Na₂ EDTA, 0.020 g. FeCl₃ ·6H₂ O, 0.00368 g. (NH₄)₆ Mo₇ O₂₄ ·4H₂ 0,0.00105 g. H₃ BO₃, 0.00040 g. ZnCl₂, lesser amounts of both CuCl₂ ·2H₂ Oand MnCl₂ ·4H₂ O, and an amount of KOH sufficient to raise the pH of thenutrient medium to within the range of 7.6-7.8. All of the aboveingredients except the KOH were combined to form a liquid nutrientmedium precursor which was introduced into the trough 13 and pipes 32,39 of the apparatus. Due to its nutrient content, the precursor wassubstantially acidic (pH 2.2) so that potentially contaminatingorganisms in the sea water, trough and pipes were killed by a period ofexposure to the acidity of the precursor.

At the completion of this sterilization step, the KOH was added to theprecursor to bring the precursor to a pH suitable for growth of thePhaeodactylum tricornutum, to complete the nutrient content of theliquid nutrient medium and to bring the level of liquid medium in thetrough up to an average running depth of 2.2 centimeters. The pH rangeof 7.6-7.8 was well within the pH range of 7.5 to 9.5 preferred formaintaining a substantial bicarbonate concentration in the sea waterbased medium. The medium was circulated by the gas lift pump for aperiod sufficient to completely mix the ingredients. Thereafter thecompleted medium 14 was innoculated with a culture of the algae.Artificially formulated sea water could have been used in place ofnatural sea water to reduce the need for sterilization steps, but it isalmost impossible to formulate sea water accurately.

The trough was located beneath a transparently bottomed tray containinga three centimeter layer of an aqueous solution containing three weightpercent copper sulfate. Using this filter nearly ninety percent ofinfrared radiation was removed from incident sunlight.

The innoculated liquid medium was continuously circulated at a mean flowrate of one foot per second. The algae were allowed to grow andreproduce and did so rapidly. Nutrients were replenished as needed.After a period of rapid reproduction, cell division rate decreased,indicating that a near maximum cell concentration in the liquid has beenreached. Thereafter, the liquid medium and entrained algae were furthercirculated and nutrients added as needed to maintain cell metabolism,but no HNO₃ was added with the makeup nutrients. The nitrogen content ofthe medium was thus allowed to become substantially depleted. In thisnitrogen-lean medium, algal cells continued to grow in size and weight,but cell division rate was greatly reduced. The amount of cell mattercontinued to increase and the lipid content of the cells increasedmarkedly. Most efficient lipid production resulted when the liquidmedium was maintained in the temperature range between 22° and 34° C.

When the concentration of algae reached approximately four grams perliter of liquid medium, about half the algae was harvested leaving abouttwo grams of algae per liter of medium. Makeup nutrients and water wereadded to the remaining liquid medium to bring its volume and nutrientcontent, including nitrogen, back up to the full strength. Celldivision, growth and harvesting proceeded continuously according to thesteps previously described.

During the entire growth period, a temperature condition was maintainedto enhance the growth and reproduction of algal cells. Temperature inthe airspace 29 was maintained at a maximum of 30° C. by circulatingcooled, filtered air through the space on warm days. The temperature ofthe liquid medium 14 was maintained between 18° and 28° C. with thehighest temperatures occurring during daylight hours and the lowesttemperatures at night. Average medium temperature during theexperimental period was 24° C.

Operating in this manner a Phaeodactylum tricornutum production ratesignificantly higher than has previously been reported for mass culturesystems was achieved. The presence of KOH in the liquid medium appearedto cause a change in the algal cell morphalogy and to cause an increasein the cells' nutrient intake. Nitrogen intake by the cells wasextremely rapid considering that the medium contained nitrogen asnitrate ions.

The source culture used to innoculate the liquid medium 14 was typicalof commonly described Phaeodactylum tricornutum cultures and containedthe typical ovate forms. However, following exposure to the nutrientmedium described, the ovate forms disappeared, leaving only the fusiformtype. In this medium, the fusiform cells enlarged greatly, attainingforty microns in cases, and the entire cell volume filled to the extentthat extension arms (normally present) were absent. Cells grown underthese conditions contained nearly four times the cytoplasm mass ofnormal fusiform cells and thus were producing nearly four times theweight for each parent cell division.

Furthermore, the cells showed an entirely new mode of cell divisionpossibly due to the high concentration of potassium ions in the liquidmedium. Large fusiform cells were observed having one or more budsgradually protruding near the apex. These buds continued to expandoutwardly until an equivalent number of legs would form. Indentation ofthe cell would continue until there were two or more cells, stillconnected apically. It was not uncommon to see three and four cellclusters radiating outward from the center where they would remainconnected. It thus appears that Phaeodactylum tricornutum may have thecapacity to produce three to four cells per cell division in a properlyselected liquid medium.

It was further observed that the normal sequence for cell enlargementduring night hours and cell division during daylight hours was reversedduring this experiment.

From the harvest results obtained in this experiment, it can beextrapolated that a yield of 38.06 ash free dry tons of algae peracre-year would be obtained when using the system described. Assumingfurther optimization, yields of seventy to ninety tons seem within reachfor certain locations and climatic conditions. The experimental resultsare far superior to those previously reported and indicate aphotosynthetically active radiation utilization efficiency rate farabove the values achieved by previous mass algal culture systems.

By the method of operation described in the above example, a liquidmedium can be continuously recycled and reused until such time as itbecomes contaminated with undesired organisms or until waste substanceswhich cannot be removed from the liquid medium accumulate to a toxiclevel.

The apparatus described can be used to culture both fresh water andmarine algae. Although Phaeodactylum tricornutum is an especiallysuitable marine species for mass culture, numerous other species couldbe reproduced by means of the present invention. Examples of suchspecies appear in the following non-exclusive list:

I. MARINE/ESTUARINE TYPES

Fragilaria sublinearis

Skeletonema costatum

Cyclotella nana

Isochrysis galbana

Pavlova gyrens

Monochrysis lutheri

Coccolithus huxleyi

Nitzschia palea

Dunaliella tertiolecta

Prymnesium paruum

II. FRESHWATER TYPES

Chlorella spp.

Chlamydomonas reinhardtii

Synedra acus

Scenedesmus spp.

Asterionella formosa

Navicula spp.

Nitzschia spp.

Fragilaria spp.

Chrysococcus spp.

Cyclotella spp.

Dinobryon spp.

Freshwater algae may be easier to maintain in a continuous process thanmarine algae because media suitable for marine algae are based onseawater which contains a very complex mixture of nutrients, many ofwhich are present in minute concentrations. It is more difficult tomonitor and maintain the chemistry of seawater based media as opposed tomedia based on freshwater.

While I have shown and described a preferred embodiment of my invention,it will be apparent to those skilled in the art that changes andmodifications may be made without departing from my invention in itsbroader aspects.

For example, troughs for containing the nutrient medium can besubstantially straight and can be constructed to sufficient length sothat algae would grow to their maximum concentration and size during onetrip through the trough. Nutrients can be injected at intervals alongsuch a trough to maintain a desired nutrient content. Heat exchangemeans can be provided at intervals along the trough to regulate thetemperature of the liquid medium.

Such a "one-pass" trough can be used to provide multiple crops, bywithdrawing a portion, e.g. fifty percent, of the liquid mediumcontaining a mature culture and replacing the withdrawn portion withfresh nutrient medium prior to the terminus of the trough. This can berepeated at intervals along the trough at any location where the culturereaches a desired state of maturity. The makeup nutrient medium can bechanged at each such location, as desired, so that different portions ofthe trough will produce algal cells having different characteristics.

Another means for obtaining cells of different characteristics from a"one-pass" trough would be to branch the trough as shown schematicallyin FIG. 12. Instead of adding makeup medium as described above, eachbranch line can be reduced in size so that it carries only a portion ofthe total culture. The trough 110 of FIG. 12 has an inlet end 114 andthree discharge ends 116, 118, 120. As liquid medium flows through thetrough it is divided into separate streams by longitudinal baffles 122.Because the different branches are of different lengths, the maturity ofthe culture removed depends on the total channel length between theinlet end and a particular discharge end. Algal cells may have differentcharacteristics depending upon the maturity of the culture so that algaeharvested from the various branches may be best suited for differinguses.

When culturing Phaeodactylum tricornutum, for example, cells removed atthe discharge end 116 might best be used as a protein source. Dependingon the branch spacing, cells taken from the discharge end 118 might bemost useful as a source of lipids and those which complete the journeyto discharge end 120 best used for the production of antibiotics.Careful regulation of nutrient conditions in each of the separatechannels could further enchance the variation among algal cells.

What is claimed is:
 1. A process for sterilization of an algal culturecontainer comprising:formulating a liquid culture medium precursor fromacidic nutrients; filling an algal culture container with the acidicculture medium precursor to sterilize said container; and adding analkaline substance to said precursor after completing sterilization toraise the pH of the medium precursor whereby a liquid medium suitablefor culture of a desired algal species is formed.
 2. The process ofclaim 1 wherein said alkaline substance comprises an alkaline nutrientwhich both raises the pH of said precursor and adds to said precursor'snutrient makeup to complete said liquid medium.
 3. The process of claim2 wherein:said precursor comprises a nutrient solution which has aboutthe composition of sea water in which is dissolved a mixture of nutrientsalts which, in grams per 100 liters of sea water, consists essentiallyof 38.0-63.4 g. HNO₃, 5.9-9.8 g. H₃ PO₄, 3.2-5.4 g. KCl, 1.40-2.33 g.Na₂ EDTA, 0.015-0.025 g. FeCl₃ 6H₂ O, 0.00275-0.00460 g. (NH₄)₆ Mo₇O₂₄.4H₂ O, 0.0008-0.0013 g. H₃ BO₃, 0.00030-0.00050 g. ZuCl₂, and lesseramounts of both CuCl₂.2H₂ O and MnCl₂.4H₂ O; and said alkaline nutrientcomprises an amount of KOH sufficient to raise the pH of said solutionto within the range of 7.6-7.8.
 4. The medium of claim 3 wherein saidmixture contains, in grams per 100 liters of sea water, about 50.7 g.HNO₃, 7.8 g. H₃ PO₄, 4.3 g. KCl, 1.86 g. Na₂ EDTA, 0.020 g. FeCl₃.6H₂ 0,0.00368 g. (NH₄)₆ Mo₇ O₂₄.4H₂ O, 0.00105 g. H₃ BO₃ and 0.00040 g. ZnCl₂.